Opal
Amorphous Hydrous Silica (Opal-A or Opal-CT) · Precious Gemstone
Crystal System
Amorphous (None). Habit: Massive, botryoidal, vein-fillings, or replacement of organic material (pseudomorphs).
Mineral Group
Silicates (Tectosilicate class, though non-crystalline/mineraloid)
Chemical Formula
SiO2·nH2O (hydrated silica with trace elements like Fe, Ca, Mg, Al)
Formation Process
Sedimentary/Hydrothermal: Silica-rich water seeps into cracks; as water evaporates, silica spheres settle into a grid that diffracts light.
About This Crystal
Small iridescent fragment exhibiting play-of-color with blue and green flashes. Typically translucent to opaque with a sub-vitreous to waxy luster.
Physical Characteristics
Color: Multicolored play-of-color; Streak: White; Cleavage: None; Fracture: Conchoidal; SG: 1.9-2.3; Fluorescence: Variable green or white.
Optical Properties
Isotropic (Singly refractive); RI: 1.37-1.47; Dispersion: High; Phenomenon: Play-of-color (iridescence) due to light diffraction from silica spheres.
Hardness & Durability
Origin Region
Australia (Coober Pedy, Lightning Ridge), Ethiopia (Welo), Mexico, and Brazil. Forms in low-temperature sedimentary or volcanic environments.
Hardness & Durability
Mohs Hardness: 5.5-6.5; Toughness: Very Poor (brittle); Stability: Sensitive to heat, drying out (crazing), and sudden temperature changes.
Care & Maintenance
Avoid ultrasonic cleaners and chemicals. Clean with mild soapy water. Store in moist cotton or away from direct sunlight to prevent dehydration.
Rarity & Value
Precious opal is uncommon. Value depends on 'fire' intensity, pattern, and base color (black being most valuable). Treatments include smoke or dye.
Special Characteristics
Play-of-color: The visible spectral colors change as the stone is tilted; sensitive to water absorption (hydrophane properties in some varieties).
Lore & History
Ancient Romans called it 'Opalus' (Precious Stone). Traditionally linked to hope and purity; modern beliefs associate it with creativity and emotional release.